Move code from the Vorbis decoder plugin.

playlist and player_control are deprecated.

Use a custom pthread_mutex_t wrapper because std::mutex adds overhead.

Fixes build problems with mingw32.

Avoid implicit copy.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().